1. Field of the Invention
The present invention relates to an antenna device.
2. Description of Related Art
An antenna device may take various shapes according to various factors, such as a frequency of a radio wave to be transmitted/received, an existence of directivity of the radio wave, and an installation location of the antenna device.
For example, as a form of a non-directional antenna device to be installed on a transportation machine such as an automobile, there is an external installation antenna device. The external installation antenna device is installed so that an antenna element is exposed to the outside (the outside of the transportation machine). In the following, an automobile is cited as an example of the transportation machine, and an antenna device to be installed in the automobile is described.
As an external installation antenna device, for example, the antenna device described in Japanese Patent Application Laid-Open Publication No. 2003-87030 may be cited. The antenna device described in Japanese Patent Application Laid-Open Publication No. 2003-87030 is a mast antenna as shown in FIG. 7.
The external installation antenna device such as the one disclosed in Japanese Patent Application Laid-Open Publication No. 2003-87030 involves several problems. For example, the antenna element exposed to the outside of an automobile is sometimes removed to be stolen by somebody. In addition, the antenna element exposed to the outside of an automobile sometimes mars a beauty of an outer appearance of the automobile.
In contrast, if the antenna device is installed in the inside of the automobile, the antenna element is not exposed to the outside. Consequently, it becomes extremely difficult to remove the antenna device from the outside of the automobile, and then a risk of being stolen is extremely reduced. Moreover, because the antenna element is not exposed to the outside, the beauty of the outer appearance of the automobile is not marred.
However, if the antenna device is installed in the inside of an automobile, problems different from those of the external installation antenna device can be produced. Hereinafter an antenna installed in the inside of an automobile is referred to as “internal antenna device” at the time of describing the details of the problems.
Not only the internal antenna device but a general antenna device needs a ground pattern having an area depending on a wavelength of a radio wave in order to stabilize properties of the antenna device sufficiently. On the other hand, because a space in the automobile is limited, an installation space of the internal antenna device is also within a limited range. Consequently, if the installation space of the internal antenna device is insufficient for the area of the ground pattern which depends on the frequency of the radio wave, it is impossible to provide in the antenna device the ground pattern which has the area depending on the wavelength of the radio wave.
If the ground pattern which has the area depending on the wavelength of the radio wave cannot be provided, an electric current (hereinafter referred to as “leakage current”) leaking into an outer conductor (for example, a cable or the like) provided to extend from the antenna device becomes large. If the leakage current becomes large, the properties of the antenna device sometimes become unstable owing to a length and/or a drawing-around of a cable (coaxial cable) connected to the antenna device, and the antenna device cannot sometimes get a sufficient gain.
Thus, the internal antenna device involves the problem that the antenna device cannot sometimes obtain a sufficient gain because the installation space of the internal antenna device cannot sufficiently be secured.
The leakage current becomes larger as the area of the ground pattern becomes smaller. In other words, the more antenna device is miniaturized, the larger leakage current becomes. As result, the properties of the antenna device become unstable, and thereby the gain of the antenna device is lowered.
On the other hand, when trying to provide a ground pattern having a sufficient area in order to stabilize the properties of the antenna device, a size of the antenna device enlarges. In this case, the installation space of the internal antenna device needs to be provided according to the enlarged size of the antenna device, and this sometimes limits a design of the automobile. In addition, because the mast antenna such as the antenna device described in Japanese Patent Application Laid-Open Publication No. 2003-87030 has the long straight antenna element, providing the mast antenna in the inside of the automobile is sometimes difficult, and the design of the automobile would be limited by necessity of arranging the antenna element.
Moreover, non-directivity is required for the antenna device installed in the transportation machine in order to secure a good reception performance independent of a positional angle of the transportation machine.